An injection mold assembly for forming internally threaded molded articles usually include a female mold which surrounds an externally threaded mold core. A mold cavity is defined between the female mold and the threaded mold core into which molten plastic is injected to form the molded article.
While the molten plastic is being injected into the mold cavity, coolant is circulated through cooling channels in the injection mold assembly to cool rapidly the molded article. Once the molded article is cooled, the injection mold assembly is conditioned to a mold open condition. During transition of the injection mold assembly from a mold closed condition to a mold open condition, the female mold and the mold core are moved relative to one another to allow the molded article to be ejected from the injection mold assembly. For the reason that the internal thread of the molded article is engaged with the threaded mold core, some parts of the mold assembly are desired to be rotatable to unscrew the internally threaded molded article.
U.S. Pat. No. 5,776,521, issued on Jul. 7, 1998, discloses a traditional mold assembly for forming threaded molded article which comprises a stationary mold core with an external threaded surface and a female mold surrounding a portion of the mold core including the threaded surface in a mold closed condition to define a mold cavity between the mold core and the female mold into which molten plastic is injected to form the molded article. A rotatable sleeve surrounds the mold core and engages with the molded article formed in the mold cavity. The sleeve carries a gear wheel which meshes with a pinion. A rack is associated with the pinion and is movable linearly to impart rotation of the pinion and in turn the sleeve. A lead screw on the sleeve engages a lead screw nut to cause the sleeve to displace axially along the mold core as the sleeve rotates. Lugs on the sleeve engage the molded article and causes it to rotate with the sleeve so that the molded article unscrews from the mold core.
As described above, because the traditional mold assembly has lugs engage the molded article, surfaces of the molded article engaged with the lugs may have flaws or even be damaged. Moreover, rotation-driving means of this mold assembly comprises a sleeve, a pinion, a rack, a lead screw, a lead screw nut, etc., such design is complicated in construct and expensive in cost.
Besides, the traditional mold assembly only adapts to forming articles whose threaded holes extend in the same direction of the mold opening. When an article has a complicated shape that its threaded hole cannot be arranged in the direction of the mold opening, as shown in FIG. 7 (the mold opening direction A is perpendicular to the article's threaded hole extending direction B), the prior art will be helpless.